Diesters of naphthalene dicarboxylic acids are useful for the preparation of a variety of polymeric materials, such as polyester and polyamide. A particularly useful diester is dimethyl-2,6-naphthalene dicarboxylate (NDC). NDC can be condensed with ethylene glycol to form poly(ethylene 2,6-naphthalate) (PEN), which is a high-performance polyester material. Fibers and films made from PEN exhibit high strength and superior thermal properties, compared to those made from poly(ethylene terephthalate) (PET). Based on these advantages, PEN is highly suitable for use in the production of commercial articles, such as thin films, which can be used for the manufacture of magnetic recording tapes and electronic components. In addition, since PEN is highly resistant to the diffusion of gases, particularly carbon dioxide, oxygen and water vapor, films made from PEN are useful for the manufacture of food containers, especially hot-fill food containers. PEN can also be used to produce reinforced fibers useful for the manufacture of tire cords.
NDC is currently produced by oxidizing 2,6-DMN to prepare a crude naphthalene dicarboxylic acid (cNDA) and esterifying the cNDA. At present, NDC is used as a major raw material for the synthesis of PEN. However, some problems are presented when NDC is used as a raw material for the synthesis of PEN, compared to when 2,6-naphthalene dicarboxylic acid (NDA) is used. Firstly, water is formed as a by-product during the condensation of NDA, whereas methanol is formed as a by-product in the case of NDC, thus risking the danger of explosion. Secondly, since pure NDC is produced by esterifying NDA and purifying the esterification product, one additional processing step is involved, compared to the use of NDA. Thirdly, the use of NDC is not suitable in view of the use of existing PET production facilities. Despite the problems associated with the use of NDC, NDC is preferentially used to produce PEN because it is still difficult to produce purified NDA having a purity necessary for the synthesis of PEN.
2,6-Dimethylnaphthalene (2,6-DMN) is oxidized to form a cNDA containing various impurities, such as 2-formyl-6-naphthoic acid (FNA), 2-naphthoic acid and trimellitic acid. Particularly, the presence of FNA in a cNDA stops the polymerization for the production of PEN, thus adversely affecting the physical properties of the final polymer (i.e. PEN). It is thus essential to remove FNA present in a cNDA, but difficulties exist in removing FNA.
Under these circumstances, much research has been conducted on methods for the removal of FNA present in a cNDA or purification of NDA. For example, NDA is produced by i) recrystallizing a cNDA, ii) oxidizing a cNDA one more time, or ii) treating a cNDA with methanol to produce NDC and hydrating the NDC. Further, purified NDA is produced by hydrogenation of a cNDA. On the other hand, many processes, e.g., solvent treatment, melting/crystallization, high-pressure crystallization and supercritical extraction, have been employed to purify NDA, but they have not been successful in producing NDA with a satisfactory purity. The processes can be employed to increase the purity of NDA but cause low yield of NDA, which makes the processes difficult to practice industrially.